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Lesson

16 Hovercraft

Floating on air

Supply list:

- Styrofoam or paper plate - Soda straw

Additional Supplies for Challenge Section:

- Modeling clay - Cardboard

- Toothpicks

O ne of the biggest barriers to efficient travel is friction. When two surfaces rub against each other frictional forces resist the movement. Many vehicles use wheels to help reduce friction. But a relatively new invention takes a different approach. Hovercraft are designed to travel over a cushion of air, thus limiting the amount of

friction. This makes hovercraft very efficient.

Hovercraft, also called ACV or air cushioned vehicles, can travel over land or water because they do not actually touch the surface when they are moving. Most hovercraft are shaped somewhat like a boat. But instead of having a hull, the edge of the craft is surrounded by a flexible rubber skirt. This skirt traps air that is pumped under the craft by large fans called turbines. The trapped air becomes compressed thus raising the air pressure under the craft and lifting it above the surface. Large fans or propellers at the rear of the craft push air backwards, which results in forward movement of the hovercraft. This movement is very smooth because the craft travels on the cushion of air beneath it. The first model of a hovercraft was built in the mid 1950’s by a British inventor named Christopher Cockerill. The first full sized hovercraft was tested in 1959. Today, many hovercraft operate as ferries to carry people and cars across the water. The largest hovercraft

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in operation weighs over 300 tons and can carry up to 400 passengers and 60 vehicles. It has 4 gas turbine engines and can travel up to 90 miles per hour (140 kph). It is the fastest large sea-going craft in the world. Hovercraft are unique because they can travel over both land and water. They are especially useful in waterways that are too shallow for regular boats to travel in. And they are ideal as ferries because they can come right up onto the land to load and unload passengers and vehicles. Hovercraft operations are treated like and operate more like an airline than a shipping line. Passenger seats are very similar to those used on passenger airlines. Also, instead of having a wheelhouse or a pilot house, a hovercraft has a cabin called the flight deck. This is where the pilot sits to drive the hovercraft. A hovercraft can float because downward moving air creates an upward force. Many aircraft also use this principle. Helicopters have rotating blades with airfoil shapes that push air downward. This creates an upward force on the blades. When this force is greater than the weight of the helicopter it will lift into the air. When the force is equal to the weight of the craft it will hover in the air similarly to a hovercraft.

The osprey is an aircraft that combines the speed of an airplane with the maneuverability of a helicopter. The rotors of the osprey can be tilted. When they are in the vertical position they act like helicopter blades, pushing air down to create lift. Once the aircraft has taken off, the rotors can be tilted forward to act as propellers to create thrust while the airflow over the wings provides the needed lift. The rotors are so long that they must be

tilted up for landing, so the craft lands vertically like a helicopter. Ospreys are used primarily as troop transports for the military and may replace helicopters in may applications. Another military vehicle that has the ability to hover and to do vertical take offs and landings is the harrier jet. The harrier has one large jet engine, but it has what is called a vectored thrust system. This system can direct the gases that come out of the jet engine in any direction. If the gases are directed downward, the plane can move vertically or even hover. If the gases are directed toward the back of the plane, it can fly forward very fast. If the gases are directed toward the front of the plane, it can even fly backwards. Credits - U.S. Navy

Photo by Photographer's Mate 2nd Class Jimmy Lee

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All of these vehicles are faster and more efficient than traditional wheeled or water vehicles because they use the power of air to lift the vehicle above the surface of the ground or water to reduce friction.

Making a Floater:

You can make a simple model of an air cushioned vehicle. Punch a hole in the center of a paper or Styrofoam plate just large enough for a soda straw. Stick a soda straw through the bottom of the plate. Place the plate on a flat surface. Blow gently in the straw and watch the plate float across the surface.

Another vehicle that floats above the water is a hydrofoil. However, a hydrofoil does not float on the air like a hovercraft. Instead, it uses Bernoulli’s Principle to create lift in the water just as airplanes do to create lift in the air. When a hydrofoil is not moving it appears to be a normal boat. But attached to the boat under the water are airfoil shaped wings. As the craft begins to move through the water, the water passing over the top of the underwater wings moves more quickly than the

water under the wings, thus creating lift. As the craft goes faster, this force is actually enough to lift the hull out of the water and the whole craft moves on the wings as it is driven forward by an engine in the water. By lifting the hull out of the water, the friction between the hull and the water is greatly reduced, so less force is needed for the craft to move through the water.

Reducing friction is important regardless of whether the craft moves through the water, on top of the water, or flies through the air. Engineers and scientists work very hard to make vehicles that have as little friction as possible. The shape and smoothness of the surface of the vehicle contribute greatly to the amount of friction it experiences. Perform the following experiment to help you visualize how water and air flow around different shapes to find the most efficient shape.

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Form modeling clay into different geometric shapes. Each shape should be approximately two inches long and ½ inch thick. Make a circle, a square, a star, and a teardrop. You can make other shapes as well if you like. Using a toothpick through the center of each shape, attach the shapes to a piece of cardboard in a row near the top edge. Place the cardboard with the shapes over a sink. Tip the cardboard up at a nearly vertical angle. Using a pitcher, slowly pour water down the cardboard so that it flows onto one of the shapes. Watch the path that the water takes around each shape. Be sure to try turning the shapes in different orientations to see which direction is most efficient. Which shapes appear to allow the water to flow most easily? Which shapes seem to hinder the flow of the water? How can you apply what you learned from this experiment to the desired shape for a vehicle?

What did we learn? What is an ACV? (Air cushioned vehicle or hovercraft) How are hovercraft different from other vehicles? (They float on a cushion of air so they can travel on both land and water.) Why are hovercraft preferable to traditional boats in certain areas? (They can travel in water that is too shallow for traditional boats. Also, they can go up onto the land for loading and unloading.) Why are hovercraft more energy efficient than many other vehicles? (They greatly reduce the friction that other vehicles experience.)

Taking it further Which of Newton’s Laws of Motion are most easily seen in the operation of hovercraft? (Newton’s Third Law – for every action there is an equal and opposite reaction.) How are harrier jets similar to hovercraft? (They both use downward moving air to create lift.)